Shower Surround & drywall transition

My husband and I stripped a 6x8 bathroom to studs/plywood. We had a contractor remove the tub and replace with a new Sterling Accord Smooth tub & shower surround (3 piece surround). The attached picture shows the placement of the new tub/surround. The tub & surround are level. The surround seems to be properly aligned to the studs with the exception of the rear panel corner where it meets the back panel. From the top inside corner to the top outside of the panel the distance from the shower panel to corresponding studs ranges from 5/8” to ¼”. How do we hang the drywall? Our dilemma is how to put up the drywall on the wall above the rear panel. I can figure how to “level” it from top to bottom, but how do we transition that piece of drywall to the remaining drywall it will but against just in front of the tub/surround? It seems like it would have to grade from 5/8” down to where it will smoothly transition to the abutted sheet, but we've never seen this before so dont know if it's doable and if so how.

Shower Surround & drywall transition

I know this is an old thread but I just installed this exact tub surround and this issue is referenced in the installation instructions. The back (part furthest from the shower head) will just have a slightly angled wall above the surround and will then be pretty much level with the wall outside the shower. After taping and mudding, you can barely tell, especially with the shower rod in the exact location.

Shower Surround & drywall transition

So did the installer fail to bring the studs plumb and flush with each other with furring strips before installing the tub and surround? Also, is this tub truly 6 ft long? The OP dexcribes a 6X8 bathroom and pictures a tub that fits into the short wall space.